﻿using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.Runtime.InteropServices;

namespace ImageCropper.Util
{
    internal abstract class Quantizer
    {
        private readonly int _pixelSize;
        private readonly bool _singlePass;

        public Quantizer(bool singlePass)
        {
            _singlePass = singlePass;
            _pixelSize = Marshal.SizeOf(typeof(Color32));
        }

        public Bitmap Quantize(Image source)
        {
            // Get the size of the source image
            int height = source.Height;
            int width = source.Width;

            // And construct a rectangle from these dimensions
            Rectangle bounds = new Rectangle(0, 0, width, height);

            // First off take a 32bpp copy of the image
            Bitmap copy = new Bitmap(width, height, PixelFormat.Format32bppArgb);

            // And construct an 8bpp version
            Bitmap output = new Bitmap(width, height, PixelFormat.Format8bppIndexed);

            // Now lock the bitmap into memory
            using (Graphics g = Graphics.FromImage(copy))
            {
                g.PageUnit = GraphicsUnit.Pixel;

                // Draw the source image onto the copy bitmap,
                // which will effect a widening as appropriate.
                g.DrawImage(source, bounds);
            }

            // Define a pointer to the bitmap data
            BitmapData sourceData = null;

            try
            {
                // Get the source image bits and lock into memory
                sourceData = copy.LockBits(bounds, ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);

                // Call the FirstPass function if not a single pass algorithm.
                // For something like an octree quantizer, this will run through
                // all image pixels, build a data structure, and create a palette.
                if (!_singlePass)
                    FirstPass(sourceData, width, height);

                // Then set the color palette on the output bitmap. I'm passing in the current palette
                // as there's no way to construct a new, empty palette.
                output.Palette = GetPalette(output.Palette);

                // Then call the second pass which actually does the conversion
                SecondPass(sourceData, output, width, height, bounds);
            }
            finally
            {
                // Ensure that the bits are unlocked
                copy.UnlockBits(sourceData);
            }

            // Last but not least, return the output bitmap
            return output;
        }

        protected virtual void FirstPass(BitmapData sourceData, int width, int height)
        {
            // Define the source data pointers. The source row is a byte to
            // keep addition of the stride value easier (as this is in bytes)
            IntPtr pSourceRow = sourceData.Scan0;

            // Loop through each row
            for (int row = 0; row < height; row++)
            {
                // Set the source pixel to the first pixel in this row
                IntPtr pSourcePixel = pSourceRow;

                // And loop through each column
                for (int col = 0; col < width; col++)
                {
                    InitialQuantizePixel(new Color32(pSourcePixel));
                    pSourcePixel = (IntPtr)((Int32)pSourcePixel + _pixelSize);
                }	// Now I have the pixel, call the FirstPassQuantize function...

                // Add the stride to the source row
                pSourceRow = (IntPtr)((long)pSourceRow + sourceData.Stride);
            }
        }

        /// <summary>
        /// Retrieve the palette for the quantized image
        /// </summary>
        /// <param name="original">Any old palette, this is overwritten</param>
        /// <returns>The new color palette</returns>
        protected abstract ColorPalette GetPalette(ColorPalette original);

        /// <summary>
        /// Override this to process the pixel in the first pass of the algorithm
        /// </summary>
        /// <param name="pixel">The pixel to quantize</param>
        /// <remarks>
        /// This function need only be overridden if your quantize algorithm needs two passes,
        /// such as an Octree quantizer.
        /// </remarks>
        protected virtual void InitialQuantizePixel(Color32 pixel)
        {
        }

        /// <summary>
        /// Override this to process the pixel in the second pass of the algorithm
        /// </summary>
        /// <param name="pixel">The pixel to quantize</param>
        /// <returns>The quantized value</returns>
        protected abstract byte QuantizePixel(Color32 pixel);

        protected virtual void SecondPass(BitmapData sourceData, Bitmap output, int width, int height, Rectangle bounds)
        {
            BitmapData outputData = null;

            try
            {
                // Lock the output bitmap into memory
                outputData = output.LockBits(bounds, ImageLockMode.WriteOnly, PixelFormat.Format8bppIndexed);

                // Define the source data pointers. The source row is a byte to
                // keep addition of the stride value easier (as this is in bytes)
                IntPtr pSourceRow = sourceData.Scan0;
                IntPtr pSourcePixel = pSourceRow;
                IntPtr pPreviousPixel = pSourcePixel;

                // Now define the destination data pointers
                IntPtr pDestinationRow = outputData.Scan0;
                IntPtr pDestinationPixel = pDestinationRow;

                // And convert the first pixel, so that I have values going into the loop

                byte pixelValue = QuantizePixel(new Color32(pSourcePixel));

                // Assign the value of the first pixel
                Marshal.WriteByte(pDestinationPixel, pixelValue);

                // Loop through each row
                for (int row = 0; row < height; row++)
                {
                    // Set the source pixel to the first pixel in this row
                    pSourcePixel = pSourceRow;

                    // And set the destination pixel pointer to the first pixel in the row
                    pDestinationPixel = pDestinationRow;

                    // Loop through each pixel on this scan line
                    for (int col = 0; col < width; col++)
                    {
                        // Check if this is the same as the last pixel. If so use that value
                        // rather than calculating it again. This is an inexpensive optimization.
                        if (Marshal.ReadByte(pPreviousPixel) != Marshal.ReadByte(pSourcePixel))
                        {
                            // Quantize the pixel
                            pixelValue = QuantizePixel(new Color32(pSourcePixel));

                            // And setup the previous pointer
                            pPreviousPixel = pSourcePixel;
                        }

                        // And set the pixel in the output
                        Marshal.WriteByte(pDestinationPixel, pixelValue);

                        pSourcePixel = (IntPtr)((long)pSourcePixel + _pixelSize);
                        pDestinationPixel = (IntPtr)((long)pDestinationPixel + 1);
                    }

                    // Add the stride to the source row
                    pSourceRow = (IntPtr)((long)pSourceRow + sourceData.Stride);

                    // And to the destination row
                    pDestinationRow = (IntPtr)((long)pDestinationRow + outputData.Stride);
                }
            }
            finally
            {
                // Ensure that I unlock the output bits
                output.UnlockBits(outputData);
            }
        }

        /// <summary>
        /// Struct that defines a 32 bpp color
        /// </summary>
        /// <remarks>
        /// This struct is used to read data from a 32 bits per pixel image
        /// in memory, and is ordered in this manner as this is the way that
        /// the data is laid out in memory
        /// </remarks>
        [StructLayout(LayoutKind.Explicit)]
        public struct Color32
        {
            public Color32(IntPtr pSourcePixel)
            {
                this = (Color32)Marshal.PtrToStructure(pSourcePixel, typeof(Color32));
            }

            /// <summary>
            /// Holds the blue component of the colour
            /// </summary>
            public byte Blue
            {
                get { return _Blue; }
                set { _Blue = value; }
            }

            [FieldOffset(0)]
            private byte _Blue;

            /// <summary>
            /// Holds the green component of the colour
            /// </summary>
            public byte Green
            {
                get { return _Green; }
                set { _Green = value; }
            }

            [FieldOffset(1)]
            private byte _Green;

            /// <summary>
            /// Holds the red component of the colour
            /// </summary>
            public byte Red
            {
                get { return _Red; }
                set { _Red = value; }
            }

            [FieldOffset(2)]
            private byte _Red;

            /// <summary>
            /// Holds the alpha component of the colour
            /// </summary>
            public byte Alpha
            {
                get { return _Alpha; }
                set { _Alpha = value; }
            }

            [FieldOffset(3)]
            private byte _Alpha;

            /// <summary>
            /// Permits the color32 to be treated as an int32
            /// </summary>
            public int ARGB
            {
                get { return _ARGB; }
                set { _ARGB = value; }
            }

            [FieldOffset(0)]
            private int _ARGB;

            /// <summary>
            /// Return the color for this Color32 object
            /// </summary>
            public Color Color
            {
                get { return Color.FromArgb(Alpha, Red, Green, Blue); }
            }
        }
    }
}